1. Field of the Invention
The invention relates to a seat back for a vehicle and a seat for a vehicle including the seat back for a vehicle.
2. Related Art
A seat back, which includes a seat back frame and a plate body mounted on the seat back frame and supporting a back portion of an occupant seated in a seat for vehicle, is known as a seat back of a seat for vehicle (for example, Japanese Patent Application Laid-Open (JP-A) No. 2009-101137). An opening, which extends in a height direction of the seat back, is formed at the plate body disclosed in JP-A No. 2009-101137. The opening is formed at a position that corresponds to thoracic vertebrae of an occupant seated in the seat for vehicle. When a back portion of an occupant is pressed against the plate body, due to the opening, a reaction force applied to the thoracic vertebrae of the occupant is reduced. Accordingly, the occupant feels more comfortable to sit on the seat for vehicle.
Meanwhile, for example, in the seat back disclosed in JP-A No. 2009-101137, if a vehicle is collided by another vehicle or the like from the vehicle rear side (collided from behind) and a load having a predetermined value or more such as a force of inertia is applied to an occupant, the waist portion of the occupant is pressed against the plate body. In general, the lumbar vertebrae of the occupant seated in the seat for vehicle are gently curved. Accordingly, if the waist portion of an occupant is pressed against the plate body, the lumbar vertebrae of the occupant are stretched upward so as to have the shape of a straight line. For this reason, the head portion of the occupant is moved relative to a headrest toward the seat height direction upper side. If the position of the head portion of the occupant is changed as described above, a timing when the head portion of the occupant is pressed against the headrest and a timing when the neck portion of the occupant is pressed against the plate body become different from each other. This causes the head portion of the occupant to swing about the neck portion as a fulcrum in the seat front and rear direction.